Fiber optic sensors are employed in a variety of applications due, at least in part, to their sensitivity and immunity to environmental conditions. More particularly, interferometric fiber optic sensors in which the velocity, phase or wavelength of the propagating light is modified in response to a sensed phenomena are employed in a variety of applications. For example, optical fiber gyroscopes are commonly employed to measure the rotation rate, such as the rotation rate of an aircraft.
By varying the construction of the fiber optic sensor, a fiber optic sensor can detect a variety of physical phenomena, including acoustic, magnetic and electric energy. In addition to these physical phenomena, it is oftentimes desirable to measure the strain imparted to a workpiece or the temperature to which a workpiece is subjected.
For example, one emerging application in which fiber optic sensors are desirably employed is smart structures. As known to those skilled in the art, smart structures generally refer to structures, such as composite structures, which incorporate interactive electrical devices for monitoring or actively controlling the performance or behavior of the smart structure. For example, a smart structure can include an electroceramic actuator which induces vibrations within the smart structure to damp or offset externally induced vibrations in the smart structure.
In order to monitor the conditions to which a smart structure is subjected, it is desirable to include a sensor, such as a fiber optic sensor, within the smart structure. For example, the fiber optic sensor can be embedded within the plies of the composite structure so as to determine the strain or temperature to which the workpiece is subjected.
In order to measure the strain imparted to a workpiece or the temperature to which a workpiece is subjected, a variety of fiber optic sensors have been developed, including fiber optic sensors designed to be embedded within smart structures. In particular, fiber optic sensors having a Bragg grating written thereon have been developed to measure the strain imparted to a workpiece. As known to those skilled in the art, a Bragg grating reflects optical signals having a predetermined wavelength. However, the predetermined wavelength of the reflected signals shifts somewhat as the sensor, and therefore the workpiece to which the sensor is mounted, is subjected to strain. This shift in the reflected wavelength is based, at least in part, upon the photoelastic and thermo-optic coefficients of the optical fiber. Thus, based upon the wavelength shift of the reflected optical signals, a fiber optic sensor can measure the strain imparted to a workpiece.
As known to those skilled in the art, however, fiber optic sensors are sensitive to temperature variations such that the shift in the wavelength of the reflected light varies not only in response to strain imparted to the workpiece, but also in response to temperature variations. Accordingly, a conventional fiber optic sensor cannot accurately determine the strain imparted to a workpiece which is disposed in an environment in which the temperature can vary if the fiber optic sensor cannot differentiate between shifts in the wavelength of reflected light due to strain and temperature changes.
Accordingly, a variety of fiber optic sensors have been developed to detect the strain imparted to a workpiece, independent of temperature fluctuations in the surrounding environment. See F. Farahi, et al., "Simultaneous Measurement of Temperature and Strain: Cross-Sensitivity Considerations", Journal of Lightwave Technology, Vol. 8, No. 2, pp. 138-42 (February 1990). For example, a fiber optic sensor having two Bragg gratings surface mounted on opposite surfaces of a bent cantilever beam was proposed to provide a thermally compensated strain gauge. See M. G. Xu, et al., "Thermally-Compensated Bending Gauge Using Surface-Mounted Fibre Gratings", Int'l. Journal of Optoelectronics, Vol. 9, No. 3, pp. 281-82 (1994).
Another fiber optic sensor for detecting the strain imparted to a workpiece, independent of temperature fluctuations, has been proposed by M. G. Xu et al. in a paper entitled Discrimination Between Strain And Temperature Effects Using Dual-Wavelength Fibre Grating Sensors, published in Electronics Letters, Vol. 30, No. 13, pages 1085-87 (Jun. 23, 1994). The dual-wavelength fiber optic sensor proposed by Xu et al. includes an optical fiber having two superimposed Bragg gratings written thereon. The two superimposed Bragg gratings are designed to reflect optical signals having two distinct predetermined wavelengths, such as 850 nanometers and 1300 nanometers. The dual-wavelength fiber optic sensor also includes a remote first and second edge light emitting diodes (ELED's) which transmit first and second optical signals, respectively, having the two distinct predetermined wavelengths through opposite ends of the optical fiber. Based upon differences in the respective wavelength shifts of the first and second optical signals as reflected by the two superimposed Bragg gratings, the strain imparted to the workpiece can be determined independent of temperature fluctuations.
As known to those skilled in the art, the light reflected by a Bragg grating of an optical fiber has different wavelength shifts at different transmitted wavelengths. This difference in the wavelength shifts increases as optical signals having larger differences in wavelengths are transmitted therethrough.
As known to those skilled in the art, an optical fiber is single mode, i.e., the optical fiber transmits optical signals in a single spatial mode, for optical signals having a wavelength within a first predetermined range of wavelengths. Consequently, the optical fiber is also typically multimode for optical signals having wavelengths outside of the first predetermined range such that the resulting optical signal transmission is less efficient. In addition, the range of wavelengths for which an optical fiber is single mode also generally decreases as the optical fiber is bent or is strained. For example, optical fibers which transmit signals in a single mode at 1300 nanometers are typically multimode for optical signals having a significantly shorter wavelength, such as 850 nanometers particularly in instances in which the optical fiber is bent or otherwise subjected to strain. Accordingly, once the dual-wavelength fiber optic sensor of Xu, et al. is subjected to strain or is bent, the dual-wavelength fiber optic sensor, which transmits and reflects optical signals having significantly different wavelengths, may be multimode for at least some of the optical signals. Therefore, the efficiency with which the dual-wavelength fiber optic sensor transmits and detects optical signals decreases which, in turn, can decrease both the sensitivity of the fiber optic sensor and the efficiency at which the multimode optical signal transmits through other optical devices, such as fiber couplers or any other fiber optic-based devices that require the light traveling through the optical fiber to be single mode.
The dual-wavelength fiber optic sensor of Xu, et al. also includes first and second fiber couplers for coupling the optical signals transmitted by a respective ELED to the optical fiber and for coupling the reflected optical signals from the optical fiber to an optical spectrum analyzer. Since optical signals having two distinct wavelengths are transmitted and reflected via the single optical fiber of the dual-wavelength fiber optic sensor, the first and second fiber couplers must generally be first and second wavelength division couplers. As known to those skilled in the art, wavelength division couplers separate optical signals based upon the respective wavelengths of the optical signals. Accordingly, reflected optical signals having different wavelengths can be separated and individually analyzed. However, wavelength division couplers are relatively expensive so as to thereby increase the expense and complexity of the resulting dual-wavelength fiber optic sensor. In addition, a wavelength division coupler cannot generally be used with multimode optical fibers.